\begin{frame} \frametitle{Single Value Subqueries} \begin{goal}{} Use constraints to ensure that the query returns only one row!\\ \remark{% The DBMS will yield a runtime error if the subquery returns two or more rows. } \end{goal} \pause\bigskip \begin{block}{} If the subquery has an \emph{empty result}, the \emph{null value} is returned. \end{block} \pause\smallskip \begin{code}{\textwidth}{Bad style!} \small \BUseVerbatim{query} \end{code} \end{frame}